Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same. The present invention is suitable for image pickup apparatuses that use an image sensor, such as a digital still camera, a video camera, a monitoring camera, or a broadcasting camera, and image pickup apparatuses, such as a camera that uses silver-halide film.
Description of the Related Art
Image pickup apparatuses, such as a digital still camera and a video camera, that use a solid-state image sensor have recently increased in performance and decreased in size. To that end, zoom lenses for use in such apparatuses are required to have a high magnification ratio, a wide angle of view, be compact, and have a high optical performance in all zoom ranges. As a zoom lens that meets such requirements, a four-unit zoom lens is known which consists of lens units having positive, negative, positive, and positive refractive power in this order from an object side to an image side.
A zoom lens disclosed in Japanese Patent Laid-Open No. 2010-256845 consists of lens units having positive, negative, positive, and positive refractive power in this order from the object side to the image side, of which a second lens unit consists of three negative lenses and one positive lens in order from the object side to the image side. Disposing the three negative lenses in the second lens unit to reduce the proportion of sharing the magnification varying operation of the individual negative lenses allows the Petzval sum to be reduced. Furthermore, disposing the three negative lens at the object side allows the principal point of the second lens unit to be close to the image side, thus reducing the distance between the principal point of a first lens unit and the principal point of the second lens unit.
In general, to obtain a compact, high-magnification image-pickup optical system, the number of lenses that constitute individual lens units is reduced while the refractive power of the individual lens units constituting the image-pickup optical system is increased. However, increasing the refractive power of the lens units increases the refractive power of the surfaces of the lenses constituting the individual lens units, resulting in an increase in the thicknesses of the lenses to ensure sufficient edge thicknesses of the lenses. This increases a front lens diameter (the effective diameter of a front lens), thus making it difficult to obtain a compact optical system. Furthermore, this causes various aberrations, such as chromatic aberration at a telephoto end, which are difficult to correct.